Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/127413
WORKBENCH The numbers game ant to know how many cubic centimeters (cc) that last bore job added to the displacement of your motor or would you like to know what the theoretical top speed of your bike is? One area that even the most knowledgeable home mechanic (and most shop mechanics for that matter) lack experience in is playing the numbers game. Calculating the displacement of your engine before and after that "big bore" installation will tell you exactly how many cc's you gained and what the new displacement is. To do it, all you need is a calculator and a workshop manual or a magazine review of your bike that lists the transmission ratios. The formula is: bore (squared) x the stroke x .7854 divided by 1000. For example, if the bore of a stock Yamaha FZR600 is 59min, multiply that times itse lf (3481) times the stroke 54.8mm (190758.8) times the constant .7854 (142267 .91) divided by 1000 . The Answer, 142.226791, is the displacement of one cylinder. Multiply that by 4 and the answer comes out 599cc. Okay let's say you had your machinist install 2mm over pistons. Using our formula, the new displacement is now 61 x 61 x 54.8 x .7854 = 160151.54 divided by 1000 and multiplied by 4 (cylinders) eq ua Is640.606cefor a net increase of 41cc. Now with the increased power from your big bore installation you should be able to pull top gear. To find your speed at redline in top gear use this handy formula: mph = rpm x effective rear wheel radius, overall drive ratio x 168. To find th e overall ratio for our FZR, which is all the internal and extern al gear ratios multi pled together , we calculate the primary ratio (the number of the teeth on th e clutch basket gear divided by the number of teeth in the primary gear ), multiply that by the internal ratio (sixth driven gear divided by the drive gear) times the final ratio, which is the number of teeth on the rear sprocket divided by the teeth of the counter sprocket. It sounds complica ted but it 's just mu ltiplying and it doesn't matter what order you do it in, but it is a job for a calculator and you will need a wo rkshop manual to get all the numbers. Bike mags often publish the numbers you 'll need when they do a full blown test or you could even get the tooth count by tearing down the transmission and actually counting them ' when you split the cases (which you'll probably be doing soon enough if you race your FZR6(0) . In any event the numbers are as follows : primary (I. 708) overall x internal (1.111) x final (3.267) = (6.1994). The numbers I'm using are for stock sprocket sizes (15/ 46). If you have changed your sprockets you will have to use new numbers. With that in mind, we need to measure the rear.wheel radius which is the distance from the ground to the center of the axle with the bike perpendicular. This number will change when you switch to a lower or higher profile tire so it follows that you can also use this formula to calculate the difference in top end gearing with differ ent profile tir es. The rear wheel radius for my rear tire is 12.5". Plugging that number along with the eng ine redline (11,000) int o the formula: mph = 11 ,000 x 12.5 divided by 1041.432 (6.199x 168), we get 132.02 mph. If you find that your fresh bore kit pu shes you deep into redline say to 12,000 rpm your top speed is now 144.03 mph. Of course, you ca n play all kin ds of ga mes with th ese numbers, like calculating the sprocket size you need to go 200 mph (31) or the revs you need to turn to run wi th a ZX-I I (14236 rpm). ClI It 's a ll in the numbers. W THE 'AWNOF A NEW ERA , GIIME:t. MOTORCYCLE 92 I,reetl February 28 ToMarch8 ~ --0 FEATURING THE 51st ~ .~ DAYTONA 200 BY ARAI ::-~ SUNDAY,-MAR€H 8 -- ALSO••• DAYTONA SUPERCROSS BY HONDA SATURDAY, MARCH 7 ARAI TWIN 50'. SUPERBIKE QUAUFIERS HARLEY-DAVIDSON TWIN SPORTS CLASSICS DAY VINTAGE ROAD RACES . DAYTONA INTERNATIONAL MOTORCYCLE SHOW AND MUCH MORE••• PLAN AHEAD BUY YOUR RESERVED SEAT TICKETS NOW! $25 - $30 Da yt ona Beach Municipa l Stadium S hort Track Races $10 - $25 W«:dnesday - Saturday, March 4 • 7 Featuri ng The.Bud weise r 60 0 cc National Friday, March 6 And The Ca mel Pro Kick-Qff Saturday, March 7 RACING ACTION STARTS AT 8 P.M . For Ticket Information Call 904-253-RACE DAYTONA INTERNATIONAL SPEEDWAY P.O. Box 2801 ~.;~!!&:_.=. Daytona Beacl~, FL 32120-2801 Maxima Tops Florida Winter National Olympics Kevin Windham 1991 WorldSports Bronze Bike Award SUPERCROSS: MOTOCROSS: 1st 12·13 MOD. Kevin Windham, Kaw, 1st \2-13 STK. Kevin Windham, Kaw, 1st \25 C Mod. Jeff Dement, Sue, 2nd 2501500 A STK, Jeff Demen t, Suz. 4th 250/500 A Mod. Jeff Dement, Suz. 1st 8512·\3 MOD. Kevin Windham, Kaw, 1st 85 \2·13 STK. Kevin Windham, Kaw, 1st 250/500 A MOD. Jeff Dement, Suz. 1st 250/ 500 A STK. Jeff Dement, Suz. 5th \25 A STK. Jeff Dement, Suz. GRAND P RIX: lst ·\2-13 STK. Kevin Windham, Kaw. 2nd \25 A STK. Jeff Dement, Suz.